1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, incorporating such a fixing device.
2. Description of the Background Art
In electrophotographic image forming apparatus, such as photocopiers, facsimiles, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical, looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image under heat and pressure.
One conventional type of fuser assembly employed in the fixing device is an endless belt looped for rotation around a generally cylindrical, stationary metal pipe, which has its outer circumference entirely or partially facing the inner surface of the looped fuser belt. The metal pipe has its circumference subjected to heating, typically with a radiant heater, from which heat is radially transferred to the length of the fuser belt rotating around the metal pipe.
Using the combination of a looped belt and a thin-walled metal pipe, the fuser assembly allows for heating the fixing nip swiftly and uniformly, resulting in shorter periods of warm-up time and first-print time required to complete an initial print job upon startup. This type of fixing device therefore has high immunity against printing failures caused by insufficient heating of the fixing nip in high-speed applications.
One problem encountered in the fuser assembly described above is that the combination of a fuser belt and a metal pipe, the former rotatable around the latter held substantially stationary, come into repeated frictional contact with each other, resulting in accelerated wear of the fuser members and high torque required to rotate the fuser belt around the metal pipe. To counteract this problem, a common practice is to provide a lubricant between the fuser belt and the metal pipe to reduce frictional resistance at the interface therebetween.
Although generally successful for its intended purposes, provision of lubricant, however, does not have a durable, long-lasting effect. This is because the lubricant tends to be squeezed out or flow away from the area of contact over the circumferential surface of the metal pipe. In particular, the lubricant can migrate to the inside of the metal pipe, where the circumference of the pipe is provided with multiple perforations to reduce heat capacity for maximizing thermal efficiency in heating the belt therearound. Such leakage of lubricant results not only in loss of lubrication and consequent increase of frictional resistance at the belt/pipe interface, but also in reduced thermal efficiency of the fixing device where the displaced lubricant adheres to the exterior of the radiant heater to affect proper radiation of heat inside the metal pipe.